1. Field of the Invention
The present invention relates to a lighting device using a light emitting element.
2. Description of the Related Art
A light emitting element is a self light emitting element and is attempted to be used as a lighting device. The light emitting element is a surface-emitting body, and a lighting device capable of emitting near-natural light can be obtained by using the light emitting element for lighting.
The light emitting element has an anode, a cathode, and a layer including a light emitting material which provides luminescence (Electroluminescence) by applying an electric field thereto. A hole injected from the anode is combined with an electron injected from the cathode in the layer including a light emitting material, thereby obtaining luminescence. Luminescence obtained from the layer including a light emitting material includes luminescence (fluorescence) that is obtained in returning from a singlet excited state to a ground state and luminescence (phosphorescence) that is obtained in returning from a triplet excited state to a ground state.
In a light emitting device using such a light emitting element, an electrode in the direction of light emission needs to be transparent. However, a transparent conductive film which is typically used as a transparent electrode often has relatively high resistivity, and voltage drop is caused in a portion away from a current supply terminal. Specifically, the lighting device is often intended to emit light from the entire surface at the same luminance; therefore, in-plane nonuniformity of luminance becomes further noticeable.
When the lighting device has large area, luminance is lowered in a portion through which current is hard to flow. In other words, luminance is nonuniform in a light emitting region of the lighting device. An ITO electrode typically used as the anode has higher electrical resistance than that of metal such as Al used as the cathode. Therefore, voltage drop is caused in a portion away from a current supply terminal, which results in low luminance. In order to solve the problem, Reference 1 reports a structure in which at least a portion of an anode is provided with an auxiliary electrode having lower electrical resistance than that of the anode (Reference 1: Japanese Patent Laid-Open No. 2004-134282).
In Reference 1, an auxiliary electrode is provided on a long side or a short side of a light emitting element. When the lighting device has larger area, luminance is lowered in a portion away from the auxiliary electrode (for example, in a central portion of the lighting device). However, the auxiliary electrode can only be partly provided, because light emitted from a light emitting layer cannot be extracted outside when the auxiliary electrode is entirely provided on the anode in the structure of Reference 1.
Since the lighting device is often intended to emit light from the entire surface at the same luminance, nonuniformity of luminance becomes further noticeable.